What’s Under the Skin?

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Fitness, it’s not the surface contours, it’s what’s next to the bone.

 

Here is an MRI cross sectional image of the thighs of three living people.

mri-cross-section-of-thigh

The smooth grayish material is muscle, the white material is fat, and the little ring in the center is how much bone material each person has.  The top and bottom images show two triathletes age 40 and 70.  The central image shows a male 4 years older than the older triathlete, and sedentary.  Notice not only how little muscle there is, but also its disorganized appearance.  Notice also how much thinner the walls of the bone are.

 

When people think about what fitness looks like, they’re often thinking about the surface contours of the body.  

 

What matters is what’s underneath the skin.

 

Two people with very similar surface contours could look very different underneath their skin.

 

In the image, there is a world of difference between the 70 year old person and the 74 year old person.  When did that difference start?  It’s safe to say that the 74 year old did not give up triathlons 4 years ago.  The changes we see in his body have probably been progressing for decades, possibly an entire lifetime.

  

The caption of the photo refers to the central individual as being sedentary.  What does that actually mean?

 

I looked into it and there are two very different definitions for “sedentary” and “physically inactive”.  Sedentary doesn’t have a standard definition as of this 2018 article.  But as it states, the root of the word is sedere, which means to sit.  One authority defines sedentary behavior as “any waking behavior characterized by an energy expenditure less than 1.5 METs (Metabolic Equivalent Tasks), while in a sitting, reclining or lying posture.”  Which means that standing is not sedentary even though it has a similar energy expenditure.

 

This tells us what a sedentary behavior is, but it doesn’t tell us how many hours a day one must engage in that behavior in order to have a sedentary lifestyle.  It’s unclear whether the mortality risk increases at 4 hours per day or 7 hours per day.  It’s also unclear how that changes if we get up periodically during a bout of sitting.

 

One thing that does seem to be pretty clear is that in questionnaires, study subjects underestimate their number of sedentary hours per day.

 

Physical inactive however has a more precise definition.  The relationship between sedentary behavior and physical activity or inactivity is that they can both be measured by a unit of energy expenditure called METs (Metabolic Equivalent Tasks).  METs measure energy expenditure, but can be substituted directly with RPE, or Rate of Perceived Exertion which measures how hard your body is working.

 

Rate of Perceived exertion is like measuring your heart rate and is considered a superior measure by some to establish a target heart rate for exercise.

 

Now that we’ve got that sticky detail out of the way here’s the definition for physical activity: 

“Any bodily movement produced by skeletal muscles that requires energy expenditure.”

 

The minimum guidelines for physical activity in order to not suffer increased health risks are as follows:  

  • at least 150 minutes of moderate-intensity activity throughout the week, 
  • or at least 75 minutes of vigorous-intensity activity throughout the week, 
  • or an equivalent combination of moderate-and vigorous-intensity activity. 
  • Activity should be performed in bouts of at least 10 minutes duration.

 

Moderate intensity activity is activity that uses 3 – 6 METs, or produces a Rate of Perceived Exertion of 4 – 5 out of ten.

Examples of that rate of activity include:

  • Walking at 4 MPH (fast enough that you have to continually remind yourself to pick up the pace and have an elevated respiratory rate)
  • Vacuuming and mopping
  • Bicycling at 10 – 12 MPH
  • Playing Doubles Tennis or Badminton

 

55 to 70% of the US population over age 65 do not meet these guidelines.

 

The good news is that moderate to high intensity exercise, and functional strength training can make positive changes to many systems of the body, not just the muscular system, starting at any age.  Studies have demonstrated that people in their 90s experience the positive benefits of training.

 

I hope that if nothing else I’ve made you just a touch curious about what you look like underneath your skin, as well as what you might look like 10 or 20 years from now.

 

 

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